Split the CTF-Writer API into IR and Writer-specific parts

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Split the CTF-Writer implementation in IR and Writer parts

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add CTF-IR clock getters, python bindings and tests

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Add event-fields getters, python bindings and tests

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>